Slipform

SCC and Formwork

We are thinking about incorporating self-consolidating concrete (SCC) into our production mixes. These mixes are much more flowable than our traditional mixes. What changes or modifications to our forms will we have to make?

This is a good question. So we posed it to John Dobbs, sales manager at Hamilton Form Co., Fort Worth, Texas.

Dobbs explains that the philosophy governing steel form design has always been based on the engineering assumption that freshly placed concrete acts like a liquid. Resultantly, engineers design the form walls to withstand a hydrostatic pressure of 150 pounds per square foot on all of the form surfaces.

With this design, forms can withstand the higher pressure that exists at the bottom of the form because hydrostatic pressure increases proportionally as the depth of the form increases. Since SCC's weight doesn't differ much from that of conventional concrete, existing and new forms should be capable of handling any normal casting situation. We've been speaking with some of our customers who are using forms that are more than 20 years old and they've verified that they have not experienced any problems,"says Dobbs.

Dobbs says that when his engineers first heard of SCC, they were concerned about potential cement paste leakage at form joints and headers. But after watching several pours during normal casting operations, the engineers report that this hasn't been a problem. ôWe think that the lack of vibration may be the reason why leakage doesn't occur,ö says Dobbs.

Dobbs does anticipate one area of potential change in form design. Some producers monolithically cast certain products with either a vertical wall or a riser with a lower horizontal section. Using SCC in this application could cause some production delays while the product's vertical portion is cast. The delay would be caused by the fact that casting crews might need to close the bottom of the hanging back pan assembly, which is normally left, open to prevent the boiling over of the SCC fluid.

Dobbs adds that, however, producers using SCC may be able to take advantage of increased labor efficiency created by faster pour rates. With less vibration, forms will be subjected to less wear and tear, resulting in longer service life.

Dobbs recommends that producers consider the whole casting operation when using SCC. Other production operations such as form cleaning, strand pulling, product header placement, and mechanized tarp roller dispensing should also be considered to complement the efficiencies developed by faster pour rates.